The invention is based on a tank venting system for an internal combustion engine as defined hereinafter. A tank venting system is already known (WO 91/12426), in which the fuel components volatilized in the fuel tank of the engine are carried via a tank venting line into an intake conduit of the engine and from there are delivered to the engine.
The introduction of the volatilized fuel into the intake conduit is advantageously performed downstream of a throttle valve disposed in the intake conduit and is reinforced by the negative intake tube pressure prevailing there. A tank venting valve and an adsorption filter are typically interposed between the fuel tank and the intake conduit in the tank venting line. The tank venting valve serves to selectively turn the tank venting on or off or to meter it and is located near the intake conduit. The adsorption filter is located between the tank venting valve and the fuel tank and adsorbs the volatilized fuel components that have escaped from the fuel tank.
The adsorption filter communicates with the atmosphere via a ventilation line, so that with the tank venting valve opened and with the aid of the negative intake tube pressure, fresh air flows through the adsorption filter, and a mixture of air and fuel is carried to the intake conduit, whereby the adsorption filter is regeneratable. The ventilation line is closeable by a shutoff valve, so that for checking whether the tank venting system is functional, a negative pressure can be built up in the fuel tank with the shutoff valve closed and the tank venting valve opened.
To avert an impermissibly pronounced negative pressure during the negative pressure check or if a functional fault occurs in the tank venting system, a protection valve is provided which, if a predetermined pressure difference between atmospheric pressure and negative pressure in the tank venting system is exceeded, opens so that a pressure equalization can take place between the atmosphere and the tank venting system. The negative pressure protection valve can be disposed in a known way at an arbitrary point upstream of the tank venting valve, for instance on the fuel tank, the tank venting line, or the ventilation line leading to the absorption filter.
A disadvantage of this kind of negative pressure protection valve is that its function may be unfavorably impaired as a consequence of soiling. Soiling of the negative pressure protection valve can for instance hinder the pressure equalization, and as a result the tank venting system can become damaged. Moreover, if the negative pressure protection valve is opened, dirt particles that penetrate the valve can prevent the negative pressure protection valve from being completely closeable once the pressure equalization has taken place, so that a leak occurs in the tank venting system. To assure that the function of the tank venting system will remain durably correct, the negative pressure protection valve, in known tank venting systems, must therefore be protected against soiling by expensive, complicated measures.